This invention relates to a facsimile apparatus utilizing a laser beam, and more particularly to a facsimile receiving apparatus using an accousto-optic modulator (AOM) cell.
A facsimile apparatus has been extensively improved by employing a laser source, and an optical modulator instead of a glow tube. Such laser beam facsimile apparatus has a number of advantageous features including long life of the light source, capability of high speed recording, a high utilization factor of the light and easiness of the deflection scanning of the light beam. As is well known in the prior art, facsimile recording systems are roughly classified into two systems. One of them is a system using digital binary signals to record white and black spot pictures. Another is a photograph transmission system in which analogue signals are transmitted to record half-tone images in the same manner as television signals. As a laser light is typically monochromatic, it is easy to form the small circular spot of high brightness by an optical element such as lens. This circular beam spot may be used to produce a scan line having a hight substantially equal to the diameter of the beam spot. However, when the recorded density of scan lines is made lower than the recorded density of each scan line in a direction parallel to the scan line or main-scanning direction on the recording medium as in an ordinary facsimile system, the shape of a beam spot becomes essential. If a beam spot is circular, the distance between the scan lines is larger than that of the case of using rectangular beam spot. To improve the recorded picture quality, it is required to reduce the distance between the scan lines. So long as the diameter of a recording beam is sufficiently small enough to be modulable in response to the picture signal, the quality of the recorded picture in the main-scanning direction depends on the bandwidth of a received picture signal without largely depending upon the characteristic of the optical system. On the other hand, the picture quality in a direction perpendicular to the main-scanning direction or sub-scanning direction is determined mainly by the characteristics of the recording system, particularly of the optical system. To improve the picture quality in the sub-scanning direction, it is essential that the intensity of the recorded scan line should be constant across the entire height of the scan line, in addition to higher density of the scan lines. Furthermore, it is also important to sharpen the edges of the scan line, namely to make it clear the boundary of each scan line. In order to obtain such intensity distribution at any tone not only for the digital recording system but also for the analogue recording system which requires reproduction of half tones, it is desirable that the intensity distribution of the recording beam spot in the sub-scanning direction is kept constant within the hight of the scan line and should have a rectangular form with sharp edges. To this end, it is necessary to provide a generally rectangular recording beam spot with a dimension transverse to the scan direction approximately equal to the scan spacing and the dimension along the scan direction somewhat smaller, and having the intensity distribution described above.
As one of the conventional method to obtain a rectangular recording beam spot, it has been known to employ a rectangular aperture smaller than the diameter of the irradiated light. This method is described on page 13 of a paper entitled "Fundamental and Application of Facsimiles" published in Aug. 1977 by the "Japanese Institute of Electric Communication". However, in order to make uniform the light distribution in the aperture, it is necessary to sufficiently expand the diameter of the laser beam at the aperture. This is because a laser beam has the Gaussian distribution in its crosssection. Therefore, this method the utilization factor of energy.
It has also been proposed a method wherein an acousto-optic modulator (AOM) cell is simultaneously driven with multiple frequencies to arrange multiple light beams in the sub-scanning direction. This method is described by P. A. Snopko in U.S. Pat. No. 3,935,566 issued on Jan. 27, 1976. Although in this method the utilization factor of light is high, it is necessary to provide numerous frequency sources thus complicating the circuit construction. In addition, an adjustment for making uniform the intensity of each beam causes mixed modulation of difracted lights in the AOM cell so that such adjustment is troublesome. Moreover increase in the number of the component parts decreases reliability.
Further, it has also been proposed to scan a laser beam transversely on the recording medium by sufficiently increasing the sweep frequency of the AOM cell than that of the facsimile signal. The laser beam spot has a diameter smaller than the height of the scan line. This method is described by A. M. Bardos in U.S. Pat. No. 3,997,722 issued on Dec. 13, 1976. Even with this method, however, the edge sharpness of each scan line on the recording medium has not been greatly improved. This is because the laser spot takes a functional form, that is the integral of the Gaussian function.
As is described above, each of the prior art has disadvantages. For this reason, it is difficult to employ a laser recording system in a facsimile apparatus requiring a high quality record in the sub-scanning direction, such as the receiving device of a photograph transmitting system. Even if the laser recording system is employed, it has been impossible to provide a high quality recording.